The present invention relates to proximity detectors, and finds particular application to motor vehicles for indicating to their drivers their proximity to objects such as adjacent vehicles when parking.
GB 2 214 290 describes a system of this kind. An infrared transmitter and an infrared receiver are attached to suitably spaced apart points on the bumper of a vehicle. The transmitter produces a divergent beam zone which we can conveniently take as roughly conical, and the receiver is sensitive to a similarly divergent zone which we can also take as roughly conical. The transmitter and receiver are skewed slightly towards each other so that the two cones cross each other. A sounder unit produces a sound of intensity dependent on the strength of the signal from the receiver.
Suppose that the vehicle is being parked against some fixed object such as another vehicle. The conical beam from the transmitter will illuminate a roughly circular area on the fixed object, and the receiver will be sensitive to light received from a roughly circular area on the object. The vehicle will initially be at a considerable distance from the fixed object, and the two circular areas on the object will have relative little overlap. The receiver signal will therefore be small. As the vehicle moves closer to the fixed object, so the two circular areas will move towards each other (reducing in size at the same time), so that the signal will increase. Eventually, however, the two circular areas will coincide, and as the vehicle continues to move closer to the fixed object, so the circular areas will gradually move apart again. The signal will therefore reduce again, reaching zero when the separation of the circular areas is complete. The driver can thus stop at an accurately controlled distance from the fixed object, by stopping as soon as the signal from the sounder unit drops in volume.
One disadvantage of this system is that if the fixed object is small, the system will be detected only if it is within the area of overlap of the two zones or cones of the transmitter and the receiver. Thus, if the range of operation is say between 30 and 5 cm, the area of object to which the system is sensitive will be of the same order of size (around 30 cm). Collisions with small fixed objects will thus be likely to occur if the object is displaced to one or other side of the system by more than say about 20 cm.
One object of the present invention is to overcome this disadvantage of the system of GB 2 214 290.